Two important concepts in understanding the modes of operation of a positioning system are "remote-positioning" and "self-positioning". In remote-positioning, a central station works out the location of the mobile. In self-positioning, the mobile works out its own location using data supplied by the station.
Two important modes of operation are radial remote-positioning and hyperbolic self-positioning.
Radial remote-positioning uses measurements of round trip time between a number of base stations and a mobile telephone. The distance between each base station and the mobile telephone can then be calculated by using the fact that radio waves propagate at the speed of light. There are a number of ways of measuring the round trip time, one way being the standard timing advance measurements made by base stations operating with the Global System Mobile (GSM) mobile telephony standard.
In operation the time-delay measurements from two base stations are transmitted via a radio link or fixed communication lines to a central station. From the time delays distances can be calculated, and using the distances it is possible to generate circles corresponding to loci of possible positions. The intersection of these loci establishes the position of the mobile telephone. There are two possible intersection points. Any ambiguity can be often resolved from a priori information. If this is not possible, a measurement from a third base station will resolve the issue. The third measurement will also allow a higher level of accuracy for the position measurement. Measurements from more than three base stations can also be combined, using standard techniques, to give more accurate measurements.
In the hyperbolic self-positioning system, the mobile telephone will compare the time of arrival of signals from three different base stations. The difference in time of arrival from two of the base stations will define one hyperbola, the time difference between another pair of stations will generate another hyperbola. The intersection of the two hyperbolas will define the location of the mobile telephone. In some cases there will be two intersections raising possible ambiguity. This can be resolved by the use of a fourth base station. This fourth base station will also allow a higher level of accuracy for the position measurement. Measurements from more than four base stations can also be combined to give more accurate measurements.
A combination of other modes is also possible. For example the round trip times could be measured at the mobile, so producing a radial self-positioning system. Alternatively the round trip time measurements could be made at the base stations, but sent to the mobile, which would then make the position calculations; an indirect radial self-positioning system. Alternatively, the base stations could co-operate to measure the time difference of arrival of the signals from a single mobile; a direct remote hyperbolic positioning system.
There are many possible uses for a system that allows accurate location of mobile telephones. These include locating people who are in distress, efficient dispatching of fleets, providing navigational guidance, recovery of stolen telephones, and giving geographically referenced information such as the location of the nearest restaurant. There have already been some attempts at implementation of systems for locating mobile phones. However, as yet there has not been a widespread commercial implementation. The major reason for this is that current solutions have technical deficiencies in the areas of coverage, accuracy, and cost.